The fitting room works fine. It throws in weight mapping for sure. You may want to check whether it is not your JCMs making havock. Wrinkles are best addressed with the smooth weight brush in the joint editor.

Sure the figure is not always the best source for a clothing rig. It's joint parameters are set to emulate the bending of skin, which is totally different from fabric. Most fabric has way less stretch ability and much enhanced buckling ability than skin has. In smooth hanging pants of non-stretchy fabric bend the knee. In the pant leg the fabric does not stretch, but the folds at the knee extend almost up to the kneecap. The skin in way hardly folds up to an knee bend angle of 90 degrees. Also the figure body shape usually has concave areas that are spanned bij fabric, so will behave differently, and vertex weights projected into that area from the different sides may be very different.

One needs a dedicated clothing rig donor.